1. Field of the Invention
This invention relates to an improved process for the conversion of hydrocarbons, and more specifically for the catalytic isomerization of olefinic hydrocarbons.
2. General Background
Olefinic hydrocarbons are feedstocks for a variety of commercially important addition reactions to yield fuels, polymers, oxygenates and other chemical products. The specific olefin isomer, considering the position of the double bond or the degree of branching of the hydrocarbon, may be important to the efficiency of the chemical reaction or to the properties of the product. The distribution of isomers in a mixture of olefinic hydrocarbons is rarely optimum for a specific application. It is often desirable to isomerize olefins to increase the output of the desired isomer.
Butenes are among the most useful of the olefinic hydrocarbons having more than one isomer. A high-octane gasoline component is produced from a mixture of butenes in many petroleum refineries principally by alkylation with isobutane; 2-butenes (cis- and trans-) generally are the most desirable isomers for this application. Secondary-butyl alcohol and methylethyl ketone, as well as butadiene, are other important derivatives of 2-butenes. Demand for 1-butene has been growing rapidly based on its use as a comonomer for linear low-density polyethylene and as a monomer in polybutene production. Isobutene finds application in such products as methyl methacrylate, polyisobutene and butyl rubber. The most important derivative influencing isobutene demand and butene isomer requirements, however, is methyl t-butyl ether (MTBE) which is experiencing rapid growth in demand as a gasoline component.
Pentenes also are valuable olefinic feedstocks for fuel and chemical products. Isoprene, which may be produced by dehydrogenation of isopentene, is an important monomer in the production of elastomers. To an increasing extent, pentenes obtained from refinery cracking units are alkylated with isobutane to obtain a high-octane gasoline component. The principal influence on trends in isopentene demand and pentene isomer requirements, however, is the rapid growth in demand for methyl t-amyl ether (TAME) as a gasoline component. This derivative is of increasing interest as restrictions on gasoline olefins and volatility reduce the utility of pentenes as a gasoline component and as ethers and alcohols are needed for reformulated gasolines with higher oxygen content. This interest may extend to hexenes and higher olefins having tertiary carbons which could be reacted to yield high-octane ethers.
Olefin isomers rarely are obtained in a refinery or petrochemical product in a ratio matching product demand. In particular, there is a widespread need to increase the proportion of isobutene, isopentene and other tertiary-carbon olefins for production of MTBE, TAME and other ethers. Catalytic isomerization to alter the ratio of isomers is one solution to this need. Since ethers must be supplied at lower cost to find widespread use as a fuel product and since isomerization competes with increased feedstock processing as a source of desired isomers, an isomerization process must be efficient and relatively inexpensive. In one aspect, a catalytic isomerization process must recognize olefin reactivity: isobutene in particular readily forms oligomers which could require a reconversion step to yield monomer if produced in excess. The principal problem facing workers in the art therefore is to isomerize olefins to increase the concentration of the desired isomer while minimizing product losses to heavier or lighter products.